A number of two wire configurations for operating solenoid valves in a sprinkler system have been suggested. U.S. Pat. No. 3,521,130 issued July 21, 1970 to W. E. Davis et al entitled "Sequential Operating System" discloses an electro-mechanically controlled pulsed d-c system. Multiwire a-c operating systems also have been in use with electro-mechanical controllers. Some of these systems have earth-grounded a common line to the sprinkler valve operating solenoids and used the non-grounded line for purposes of switching the valve solenoids on and off in predetermined timed sequence in response to the controller program. These multiwire systems, i.e. systems wherein each electro-mechanical valve is operated by its own controlled power wire, usually have a common wire which is earth-grounded. In such systems, however, the practice has been to reference the electro-mechanical or electronic controller to the earth-ground. This subjects the controller to the full impact of lightning-induced transient voltages.